Convert to Sheet Metal takes bent parts as modelled: radius and chamfer bends, holes (#1314)

A Convert to Sheet Metal bend point can now lie on a bend modelled with its radius (a cylinder tangent to two flat faces) or in the middle of a chamfer; either becomes one bend at the faces' virtual sharp, with the rule's radius. Holes and engravings carry over. Bend points on edges convert as before. Recorded under format 21. Tests in cmd::sheet::convert_tests; the sheet-metal guide is updated.

docs/book/src/guide/sheet-metal.md

@@ -127,14 +127,37 @@ keeps its envelope and the flat pattern is the outside lengths less a bend deduc
 2·(R + T)·tan(θ/2) − θ·(R + K·T). A bend can be picked on either skin of a shell; bends turn
 either way, follow one another and take any angle (the walls of a tapered tray at 72°). An
 edge between two faces of the sheet that is no bend is cut open, each side half the rule's
-**gap** from the plane halfway between the two, like the corners of a tray. The result is an
+**gap** from the plane halfway between the two, like the corners of a tray. A bend drawn as a
+chamfer (a flat strip between two faces, the chord of the bend's arc, as models without bend
+radii draw one) is picked in the middle of the strip: the two faces then meet at their virtual
+sharp in one bend of the rule's radius, and the strip is no face of the sheet. A bend modelled
+with its radius (a cylinder between two flat faces tangent to it, as sheet metal parts from
+other systems come, or a filleted edge) is picked anywhere on its cylinder, on either skin, and
+converts the same way: the sheet is bent there with the rule's radius and K-factor, not the
+modelled radius, so the flat pattern is the one the rule makes. The result is an
 ordinary sheet body: edge flanges, cuts, Unfold, the flat pattern and its exports work on it,
 and it follows the solid when a parameter changes (the edges and the fixed face are found by
-their names). Without bend edges Convert expects a plate, as before; its round walls (holes, slot
-ends, rounded corners) stay circles and arcs in the flat pattern and the DXF, also where a STEP
-file writes one in pieces. Not converted yet: curved faces (rolled sheet, a filleted edge as a
-bend), faces with holes or curved edges in a body with bend edges (cut them after converting)
-and bends that close a ring.
+their names). Without bend edges Convert expects a plate, as before. Either way round walls
+(holes, slot ends, rounded corners, notches) stay circles and arcs in the flat pattern and the
+DXF, also where a STEP file writes one in pieces, and the holes and slots of every face of the
+sheet are its cut-outs, where the bends put them. An engraving in a face of the sheet (a recess
+with a flat floor, at most half the thickness deep) becomes a pocket of it, as a plate's does
+(below), on either skin of a shell. Not converted yet: curved faces other than bends (rolled
+sheet), with bend edges a hole or an engraving that reaches into a bend or a pocket deeper than
+half the thickness (cut them after converting), and bends that close a ring.
+
+A plate may carry recesses in one of its faces, such as an engraved label or a counterbore: a
+recess with a flat floor and walls square to the plate, at most half the plate's thickness
+deep. Each becomes a pocket of the sheet (see the pockets below), so the converted part keeps
+it: the sheet is as thick as the plate, the flat pattern is the whole plate with the recess
+listed under `pockets` (its outline, side and depth), and the DXF draws its outline on the
+pocket layer of its depth, apart from the cut contours, for a laser to mark rather than cut.
+The face the recesses are in can be the one picked or the other. A recess deeper than half the
+thickness, recesses in both faces, and a plate whose recesses are larger than the face around
+them (a thinner plate with raised parts) are refused with the reason. A recess lies in an
+opening of the face it is cut into, so the faces of a bent part's legs are none: a bent part is
+refused as no plate, and converts with its bend edges. A bend across a pocket is refused as for
+any pocket, so fold or flange clear of an engraving.
 
 ```json
 {"command":"solid.box","params":{"length":100,"width":60,"height":30,"body_name":"Tray"}}